The present invention relates to an improved electrostatographic apparatus such as an electrostatic copying machine comprising improved developing means.
In such a copying machine a light image of an original document is radiated onto a photoconductive drum, belt or the like through three respective primary color filters. After each imaging operation a toner of the respective complimentary primary color is applied to the drum to form a toner image which is transferred to a copy sheet. The three toner images in registration on the copy sheet form a full color copy. In some copying machines, a fourth color, black, is also provided.
The three toners are applied to the drum by three respective developing units. Each developing unit comprises a container for the toner and a magnetic brush unit for applying the toner to the drum. Each magnetic brush unit comprises a rotary non-magnetic cylinder and a plurality of magnets disposed inside the cylinder. Rotation of the cylinder in combination with the magnetic force of the magnets causes toner to adhere to the outer surface of the sleeve in the form of a brush which engages the drum. The toner adheres to the high electrostatic charge areas on the drum to form the toner image.
Color electrostatic copying machines of this type have heretofore suffered from several disadvantages regarding the developing apparatus. Prior art machines comprise three complete magnetic brush developing units. These units may be fixedly mounted in a circumferentially spaced arrangement about the drum. This arrangement is disadvantageous in that a large portion of the circumference of the drum is required to accommodate the developing units and therefore the drum must be larger than is required for a monochromatic copying machine. Also, means must be provided to individually actuate the developing units at the proper times while maintaining the other developing units inoperative. A copying machine of this type must be cleaned frequently since toner from the developing units tends to be scattered about the inside of the housing. As yet another drawback such a copying machine must be necessarily large in size to accommodate the three developing units and therefore expensive to manufacture.
Another type of prior art color copying machine does not fixedly mount the three developing units about the circumference of the drum but instead movably mounts them so as to be individually movable into close proximity with the drum for operation. This type of apparatus is disadvantageous in that the gap between the sleeves of the developing units and the drum is critical and is difficult to correctly attain with the three developing units being movably mounted. Also, in order to move the developing units at a reasonably high speed the drive means must be large in size and consume a large amount of electrical power. In addition, the overall dimensions of the copying machine must be large in order to provide sufficient space for movement of the developing units.